Abstract
HIGHLIGHTS: Sedimentation of fine sediment particles onto seagrass leaves severely hampers the plants' performance in both light and darkness, due to inadequate internal plant aeration and intrusion of phytotoxic H2S.Anthropogenic activities leading to sediment re-suspension can have adverse effects on adjacent seagrass meadows, owing to reduced light availability and the settling of suspended particles onto seagrass leaves potentially impeding gas exchange with the surrounding water. We used microsensors to determine O2 fluxes and diffusive boundary layer (DBL) thickness on leaves of the seagrass Zostera muelleri with and without fine sediment particles, and combined these laboratory measurements with in situ microsensor measurements of tissue O2 and H2S concentrations. Net photosynthesis rates in leaves with fine sediment particles were down to ~20% of controls without particles, and the compensation photon irradiance increased from a span of 20–53 to 109–145 μmol photons m−2 s−1. An ~2.5-fold thicker DBL around leaves with fine sediment particles impeded O2 influx into the leaves during darkness. In situ leaf meristematic O2 concentrations of plants exposed to fine sediment particles were lower than in control plants and exhibited long time periods of complete meristematic anoxia during night-time. Insufficient internal aeration resulted in H2S intrusion into the leaf meristematic tissues when exposed to sediment resuspension even at relatively high night-time water-column O2 concentrations. Fine sediment particles that settle on seagrass leaves thus negatively affect internal tissue aeration and thereby the plants' resilience against H2S intrusion.
Highlights
Anthropogenic activities in coastal waters such as dredging, point-source outfall discharges and runoff from agricultural and urban catchments lead to addition and resuspension of fine particulate material that can have substantial negative impacts on the health and fitness of seagrasses (Erftemeijer and Lewis, 2006 and references ; York et al, 2015; Chartrand et al, 2016)
As we introduced a physical barrier to O2 diffusion at the abaxial surface by fixing the leaf onto polystyrene with a low O2 permeability, we take the flux estimated at the adaxial side of the seagrass leaf as representative for the net flux of O2 across the leaf surface, i.e., JO2, tot = JO2, upper-surface in dark (=respiration) and light (=net photosynthesis; assuming a photosynthetic quotient of 1 mol O2 produced per mol CO2 fixed), respectively
Our results provide strong evidence that silt/clay-cover on seagrass leaves can have substantial negative effects on the plants’ photosynthetic activity and efficiency, as well as on the nighttime O2 exchange between leaf tissue and the surrounding water
Summary
Anthropogenic activities in coastal waters such as dredging, point-source outfall discharges and runoff from agricultural and urban catchments lead to addition and resuspension of fine particulate material that can have substantial negative impacts on the health and fitness of seagrasses (Erftemeijer and Lewis, 2006 and references ; York et al, 2015; Chartrand et al, 2016). The indirect effects associated with turbid sediment plumes, have largely been attributed to reduced light availability impeding seagrass photosynthesis (e.g., Erftemeijer and Lewis, 2006; York et al, 2015). Dredging-induced seagrass mortality depends on the nature of the dredging operations including the duration and intensity (Erftemeijer and Lewis, 2006; York et al, 2015), but some larger dredging activities result in widespread sediment plumes that can significantly reduce light transmission through the water-column (Cutroneo et al, 2013). Seagrasses generally have high light requirements and are typically found in waters, where at least 10% of incident solar irradiance reaches the seagrass leaf canopy (Duarte, 1991)
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